ASSISTANT PROFESSOR XUMING ZHANG
Waste not, want not
An expert in photonics, Assistant Professor Xuming Zhang is applying his expertise to water process
technology. Here, he describes his fortuitous entry into the field, the principles of photocatalysis and the
particular importance of this work in his native China, where access to clean water is a significant problem
How has your career thus far prepared you
for your current studies?
My PhD and postdoctoral studies focused on
photonics and microsystems. But when I took
an academic position five years ago, I started
to look for new research directions, something
very new and critical to human society in the
future. By chance, there was a wastewater
treatment plant near my house that I passed
by every day. Looking at the many pools taking
up large areas of land, I started to think about
how I could apply my knowledge and experience
to contribute to water process technology.
Following the technical route, I have made
important progress in the last few years.
Could you introduce your work on
microfluidic planar reactors?
Microfluidic planar reactors apply microfluidics
technology to tackle the limitations of current
photocatalytic reactors. We made the first
device in 2010, which performed surprisingly
well. It was well received by the community
and the media, which encouraged us to develop
more advanced versions to solve different
physical and material problems.
Why is titanium dioxide (TiO2) a particularly
useful catalyst in this context, and what
role do microvessels play in improving a
reactor’s efficiency?
TiO2 is almost the perfect photocatalyst. It is
highly efficient, stable, has a long lifetime, and
is naturally abundant, non-toxic and cheap.
Unfortunately, it has an Achilles’ heel – it
absorbs only UV light (wavelength <380 nm)
and has almost no response to visible and
near-infrared light, which makes up more than
80 per cent of the energy in sunlight. Enormous
research efforts have been devoted to make TiO2
responsive to visible light.
One of the biggest problems of existing solar
reactors is their low efficiency. This is determined
by several factors, such as size of the reaction
surface area relative to sample volume; how fast
contaminants can be diffused to the reaction
sites, be decomposed by the oxidative ions
and then be removed from the reaction sites;
how uniformly photons are delivered to the
photocatalysts; and how efficiently photo-excited
electrons and holes are generated and separated.
Microfluidic planar reactors provide a solution to
most of these problems; they have a thin layer of
photocatalyst at the bottom, a microstructure
layer in the middle and a transparent cover. The
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INTERNATIONAL INNOVATION
reactor utilises the microstructures to uniformly
flow a thin layer of water sample over the
photocatalyst film in a controllable manner.
What was the rationale behind your
most recent project to build a pilot highthroughput photocatalytic water purification
plant using this technology?
Pilot plants using various sunlight reactors
attracted a surge of research in the 1990s.
However, most of them were inefficient and
utilised only UV light, preventing real industrial
application. Our work aims to tackle these
problems by incorporating microfluidics
technology and the plasmonic effect into pilot
plants. Structural design guided by microfluidics
will improve efficiency, while the plasmonic
effect enables the utilisation of both the UV and
visible parts of sunlight.
Is this technology scalable? Ultimately, what
kind of capacity could it have for treating
high volumes of water in an industrial setting,
for example?
Previous microfluidic planar reactors had very
limited process volume (~1 mlh-1) due to small
device dimensions. But with the successful
experience of small reactors, now we dare to
scale up microreactors into a pilot plant for large
process volume (targeted at 1,000 lh-1).
Upcoming
event
Optofluidics 2014
28-30 August
Guangzhou, China
www.optofluidics.cn
Are there practical obstacles currently
obstructing the purification of wastewater?
Why is this a particularly important issue
in China?
The primary obstacles are the immaturity and
high cost of the technology. Water purification
plants that successfully produce clean water at
an affordable cost are still lacking.
The purification of wastewater is particularly
important in China because rapid urbanisation
demands a greater supply of clean water and at
the same time produces more wastewater. In
addition, the natural water resource is limited
and severely polluted. Water purification
enables us to recycle and reclaim wastewater
and immediately boosts the supply of
clean water.
Who do you collaborate with? How
important are these partnerships to
your research?
We collaborate with Dr Weixing Yu, an expert
in plasmonic nanomaterials and nano-optical
devices. His work increases the efficiency of
sunlight absorption and our work aims to
utilise the absorbed energy to decompose
the contaminants more efficiently. This
collaboration draws on the strengths of both
teams, complementing each other’s efforts well.
ASSISTANT PROFESSOR XUMING ZHANG
Widening water supply
A team from The Hong Kong Polytechnic University is developing a novel method of purifying water by
passing it through microchannels exposed to light. If successfully scaled up, this technology could represent an
energy-efficient solution to the water shortage problem afflicting many communities around the world
AN ABUNDANT AND clean water supply is
a major challenge for modern human society.
Due to rapid urbanisation, growing population
and climate change, lack of access to potable
water is particularly felt in developing countries
like China where natural water resources are
limited and severely polluted; a problem further
compounded by competing agricultural and
industrial needs.
Although treatment can remove most
impurities, residual contaminants remain. At
present, this water is run into rivers, lakes and
seas for natural purification – both a waste of
a precious resource and a threat to the natural
environment. In this context, photocatalytic
purification – a light-induced chemical process
based on the interaction between photons
and semiconductor catalysts – offers a
promising solution.
Xuming Zhang, Assistant Professor in the
Department of Applied Physics at The Hong Kong
Polytechnic University, is working to improve
wastewater purification. He is currently leading
an innovative project that aims to leverage
microfluidics and plasmonic technology in order
to initiate high-performance, sunlight-driven
water purification on a large scale.
OPTOFLUIDICS
Photocatalysis relies on a simple principle: when a
catalyst absorbs photons with energy either equal
to or greater than the band gap – the energy range
where no electron states can exist – electron-hole
pairs are formed. These pairs proceed to react
with harmful molecules in wastewater, creating
innocuous products such as carbon dioxide.
Photocatalysis by direct sunlight has long been
heralded as the future of water processing, as it
can degrade a wide range of pollutants without
the need for electricity, but has not yet reached
its full potential. In recent years though, a
particular form of photocatalysis – plasmonic
photocatalysis – has made huge progress.
Based on the use of noble metal nanoparticles
dispersed in semiconductor catalysts, it enhances
photocatalytic efficiency under visible light.
In parallel, microfluidics is accelerating
rapidly. This multidisciplinary field offers novel
Scientific outcomes
• Studies of the limiting factors of
photocatalysis and plasmonic enhancement
mechanisms will accelerate understanding
• Novel plasmonic light-harvesting materials
and anti-reflective nanostructures will
enhance the utilisation of solar energy and
could even benefit other technologies,
such as solar cells and water splitting
• Fabrication and integration techniques
used for the high-throughput pilot plants
could pave the way for large-scale water
processing plants
solutions to the handling of fluid in microscale
environments. Already in wide use in bioanalysis
and drug discovery, microfluidics also has a
number of benefits for photocatalysis, including
large surface areas, short diffusion lengths and
short reaction times.
In recognition of this, Zhang is exploiting the
synergy of the two fields to purify treated
wastewater. By photocatalysing microchannels
of water, he hopes to bring the treated
wastewater up to the standard of drinking
water. Importantly, the technique will not
directly compete with existing methods, instead
working as a complement.
A WORLD FIRST
Photoelectrocatalytic microreactor inhibits electron-hole recombination, selectively controls the reaction pathways,
and highlights the synergetic effect of photocatalysis and electrocatalysis.
Published in Biomicrofluidics in 2010, Zhang’s
team was the first to report the successful
development of a microfluidic planar
reactor. Their reactor was an astounding
100 times more efficient than conventional
Dr Weixing Yu.
WWW.RESEARCHMEDIA.EU 67
INTELLIGENCE
HIGH PERFORMANCE SUNLIGHT
DRIVEN WATER PURIFICATION PILOT
PLANT BASED ON PLASMONIC
PHOTOCATALYSTS AND MICROFLUIDIC
PLANAR REACTORS
OBJECTIVES
• To develop next-generation photocatalytic
reactors by intelligently integrating
microfluidics and plasmonic photocatalysis
to solve the major problems in traditional
photocatalytic water purification such as
mass transfer limit, photon transfer limit
and low photonic efficiency
• To develop novel large-area sunlightresponsive plasmonic photocatalytic
nanomaterials to meet high efficiency and
cost-effectiveness requirements of new
photocatalytic reactors
• To investigate high-performance
light harvesting and anti-reflection
nanostructures to enhance the utilisation
of sunlight
• To develop large-scale microfluidic-based
plasmonic photocatalytic reactors for high
process capacity and high purification
efficiency under sunlight
KEY COLLABORATOR
Dr Weixing Yu, State Key Laboratory of
Applied Optics, Changchun Institute of
Optics, Fine Mechanics and Physics, Chinese
Academy of Sciences
KEY TEAM MEMBERS
Ning Wang • Furui Tan • Xiaowen Huang •
Chi Chung Tsoi
FUNDING
National Natural Science Foundation of
China (NSFC) grant no. 61377068
Research Grants Council (RGC) of Hong Kong
grant no. N_PolyU505/13
CONTACT
Assistant Professor Xuming Zhang
Principal Investigator
Department of Applied Physics
Hong Kong Polytechnic University
Hung Hom, Kowloon, Hong Kong
T +852 6732 1734
E [email protected]
XUMING ZHANG is currently Assistant
Professor within the Department of Applied
Physics, Hong Kong Polytechnic University.
He received a BEng from the University of
Science & Technology of China in 1994, and
PhD from Nanyang Technological University
in 2006. His research interests include
microfluidics, photonics and green energy.
photocatalytic reactors. According to Zhang,
this was the result of four unique features:
“First, a thin layer of water ensures the
diffusion time of the contaminants to the
reaction surface is short and the titanium
dioxide (TiO2) photocatalyst film provides
a large reaction surface area. Second, the
microstructures enable uniform flow of water
over the reaction surface, ensuring the same
level of photodegradation throughout the
sample. Third, the flowing water helps to
move the reaction products in the sample,
allowing reaction sites to be open for later
reactants. And fourth, the planar design of the
reactor results in almost uniform irradiation
over the whole reaction surface and thus high
photon efficiency”. Combined, these features
radically increase the reaction rate constant
of photodegradation.
This proof of concept was met with much
excitement by the scientific community.
Building on the success, the team tested four
further designs before deciding on the final
version in 2012: a photoelectrocatalytic (PEC)
microreactor, which combines photocatalytic
and electrocatalytic effects – catalysis
taking place at electrode surfaces. The PEC
microreactor increased photoactivity and,
having been tested over 200 times, showed little
performance degradation. Furthermore, it was
shown that the device overcomes the problems
of bulk reactors, such as oxygen deficiency and a
lack of reaction pathway control.
INTERNATIONAL INNOVATION
Project trajectory
Task 1. Microfluidic reactors: investigating
the limiting factors of photocatalysis
to optimise reactor performance
using microfluidic structures
Task 2. Plasmonic nanomaterials:
developing TiO2 plasmonic
photocatalysts to absorb the UV
and visible region of the solar
spectrum, which could enhance
the use of solar energy by a factor
greater than 10
Task 3. Optical nanostructures: structuring
plasmonic photocatalysts into
nanopillar arrays
Task 4. Pilot plants: using these new
reactor designs and advanced
nanomaterials, the final task will
develop a pilot plant for highperformance, high-throughput,
sunlight-driven water purification
MAXIMISING ABSORPTION
Indeed, photocatalytic purification delivered
using microfluidics has many advantages over
conventional purification methods. For instance,
the method decomposes contaminants without
the need for toxic chemicals. Furthermore, it
can directly use sunlight, making it remarkably
energy efficient. “More importantly, the
photocatalytic reaction is nonspecific and
therefore effective on a broad range of
contaminants such as bioparticles, organic
chemicals and heavy metal ions,” Zhang adds.
However, the technology is not without its
flaws. In order to make the greatest possible
use of energy from sunlight, the photocatalyst
must be responsive to visible light. At present,
photocatalysis only works effectively under UV
light, which contains only about 3-5 per cent of
sunlight’s energy. Hence, in order to maximise
Gold nanopillars to absorb all visible sunlight.
68
Group photo with the solar reactor.
Left to right: Mr Ning Wang, Assistant Professor
Xuming Zhang, Ms Furui Tan and Ms Xiaowen Huang
this range, collaborator Dr Weixing Yu from
Changchun Institute of Optics, Fine Mechanics
and Physics, Chinese Academy of Sciences,
developed light-absorbing arrays of gold ‘nanopillars’. Data showed that such arrays could
absorb a massive 96 per cent of light, covering
the whole visible region. Such high absorption
will be critical for their proposed pilot plant, and
the wide receiving angle of the pillars (±60°) will
allow operation in diverse weather conditions.
TECHNOLOGY FOR THE PEOPLE
Looking forward, in the short term the team
hopes to fabricate large photocatalyst plates
and develop pilot plants based on these. This will
ultimately lead to high-performance, sunlightdriven plants for high-throughput water
purification, enabling the recycling of treated
wastewater, rapidly and dramatically boosting
water supply to China. If Zhang were able to
purify just half of wastewater in China, the
total water supply would be doubled, bringing
significant social and economic benefits.
This technology could one day find applications
in other countries, and even in other fields.
Microreactors have potential for protein
cleavage and photosynthesis, and bulk reaction
systems have used photocatalysis to destroy
bacteria and viruses, inactivate cancer cells and
fix nitrogen.